1. Field of the Invention
The present invention relates to a capacitive humidity sensor, which has a capacitance adjusting film in a reference portion in order to reduce offset voltage of the sensor.
2. Description of Related Art
A capacitive humidity sensor, which detects humidity based on a change of capacitance between two detection electrodes provided on a semiconductor substrate, is disclosed in U.S. Pat. No. 6,580,600 B2 (corresponding to JP-A-2002-243690).
This sensor has two detection electrodes, which oppose each other, on a first insulation film formed on a surface of a semiconductor substrate. The detection electrodes are covered with a second insulation film and are further covered with a moisture sensitive film thereon. In addition, a reference portion, having a reference capacitance which does not change even when humidity changes, is provided on the semiconductor substrate.
The detection electrodes and a circuit element portion including the reference portion construct a switched capacitor (SC) circuit. The SC circuit converts a change of capacitance between the detection electrodes to a voltage signal and outputs it. Accordingly, humidity can be detected based on a difference between the reference capacitance and a capacitance of the detection electrodes, which changes according to humidity.
In this capacitive humidity sensor, the reference portion includes the semiconductor substrate and a wiring electrode provided on the semiconductor substrate. The first insulation film, whose permittivity is different from that of the moisture sensitive film, is disposed between the semiconductor substrate and the wiring electrode. In this case, an initial capacitance difference, that is, a difference between the reference capacitance and the capacitance of the detection electrodes in a reference humidity condition (e.g., 0% RH or 100% RH) is large. This causes offset voltage. When this offset voltage is large, an output range based on a humidity change is reduced in a whole output range of the sensor. As a result, accuracy of humidity detection deteriorates. Therefore, the offset voltage needs to be reduced as much as possible.
However, troublesome countermeasures are required in order to reduce the offset voltage in this capacitive humidity sensor. For example, a pattern of the wiring electrode in the reference portion must be enlarged in order to increase an area where the reference portion faces the semiconductor substrate. Otherwise, an offset compensation circuit must be provided in the circuit element portion in order to compensate for the offset voltage. These countermeasures require an additional design of the wiring electrode or the offset compensation circuit. Further, it is difficult to reduce size of the sensor because an area where the circuit element portion is formed is enlarged.